Phase stability of materials in sodium-rich silicates at high pressure

Sodium (Na), silicon (Si), and oxygen (O) are main abundant elements to form the minerals inside the Earth's interior. Here, combining first-principles calculations and crystal structural prediction methods, we propose the formation of a distinct stoichiometry, ${\mathrm{Na}}_{6}{\mathrm{SiO}}_{5}$, and two high-pressure phases of ${\mathrm{Na}}_{4}{\mathrm{SiO}}_{4}$ ($Cmca$) and ${\mathrm{Na}}_{2}{\mathrm{Si}}_{3}{\mathrm{O}}_{7}$ ($C2/m$), all exhibiting thermodynamic and dynamical stability at extreme conditions relevant to the Earth's mantle. ${\mathrm{Na}}_{6}{\mathrm{SiO}}_{5}$ is stable over a broad pressure range (66--150 GPa) and high temperatures (up to 4000 K), with unique polyhedral configurations, and strong ionic and covalent bonding. ${\mathrm{Na}}_{6}{\mathrm{SiO}}_{5}$ and $Cmca\text{\ensuremath{-}}{\mathrm{Na}}_{4}{\mathrm{SiO}}_{4}$ retain their solid-state character at temperatures up to 4,000 K, whereas the $P\overline{1}$ and $Cc$ phases of ${\mathrm{Na}}_{4}{\mathrm{SiO}}_{4}$ undergo phase transitions to superionic or liquid states as temperature increases. $P\text{waves}$ (${V}_{P}$) and $V$ waves (${V}_{S}$) of ${\mathrm{Na}}_{6}{\mathrm{SiO}}_{5}$ and ${\mathrm{Na}}_{4}{\mathrm{SiO}}_{4}$ match trends observed in the Preliminary Reference Earth Model, suggesting that these compounds may contribute to the seismic anomalies observed in the core-mantle boundary (CMB). These findings provide theoretical insights into the potential role of Na-rich silicates in the Earth's deep interior and their influence on the dynamics of ultra-lowvelocity zones and large low-shear-velocity provinces.